Chemical and physical properties of tribofilms formed by the interaction of ashless dithiophosphate anti‐wear additives

Abstract

Physiochemical properties of tribofilms generated from octadecylthioperoxydiphosphate (SSC18) and octadecylphosphorofluoridothioate (PFC18) were compared with zinc dialkyl dithiophosphates (ZDDP) in this study. PFC18 and SSC18 were chemically synthesised and mixed in mineral base oil to prepare test oil formulations. Additionally, PFC18 and SSC18 were mixed at different concentrations and evaluated for their compatibility and tribological performance using high frequency reciprocating rig (HFRR) tribometer. All lubricant formulations with ashless additives exhibited superior anti-wear characteristics compared to ZDDP. Comprehensive surface analysis of tribofilms was conducted using scanning electron microscopy, nano-indentation and X-ray absorption near edge spectroscopy. Formulations containing only SSC18 and PFC18 exhibited a uniform and continuous tribofilm formation, while formulations with a combination of SSC18 and PFC18 had a stable pad-like structure. X-ray absorption near edge structure (XANES) results indicate ashless PFC18 and SSC18 by themselves and in combination form iron phosphate enriched tribofilms with a mixture of iron sulfides and/or sulfates.

XANES Study of Tribofilm Formation With Low Phosphorus Additive Mixtures of Phosphonium Ionic Liquid and Borate Ester

“…XANES P L-edge indicated that the tribofilms formed with P_DEHP(350P/700P)+SB contains relatively longer chainlength phosphates in the near-surface region than the bulk of the tribofilms. Earlier studies have shown that longer chain phosphates yield better tribological properties and are generally considered to be more beneficial (Bancroft et al, 1997;Mosey et al, 2006;Sharma et al, 2015;Vyavhare et al, 2021a,b,c, Vyavhare et al, 2021. Additionally, XANES results indicated that the chain length of phosphates increases with the rubbing time.…”